Conventional settler design typically collects product overflow in an overflow system, such as a launder about the perimeter of the settler vessel, for transfer to the next unit operation. In the event the next unit operation is a pump, either a pump box or capacity in the overflow system provides surge capacity for the pump to accommodate and control flow variations. In some cases, flow variations in the overflow system are not important. However, in some bitumen froth treatment methods, the overflow stream contains components which may precipitate with minor temperature or concentration variations. For example, if alkane (paraffinic) solvents are used to treat the bitumen stream, the overflow stream will be a solution containing asphaltenes which are susceptible to precipitation with changes in temperature or concentration. While the overflow stream solution may be stable at high flows, at low flows stream temperatures may cool resulting in asphaltenes depositing and fouling the overflow system. This also holds for systems with trace components that may cause fouling at low flows.
To address this problem, conventional settlers may be periodically removed from operation to clean the overflow system which is often a launder. The cleaning operation involves personnel and equipment entering the settler vessel after solvent vapours are reduced to a safe level. As the solvent commonly complexes in asphaltene deposits, bringing solvent vapour levels down to a safe level involves repeated flushings and purgings with the on-going risk that removing asphaltene deposits cause high solvent vapour levels.
An alternative approach involves stabilizing the overflow stream composition. The overflow composition from the settler is at the solubility limit of asphaltenes in diluted bitumen. Using solubility equilibrium data to either add maltenes and/or adjust overflow temperatures, the overflow stream composition can be stabilized by shifting the composition away from the solubility limit. While the solubility response of asphaltenes is relatively well known, Imperial Oil has recently filed a Canadian Patent application no. 2,669,380 to recycle bitumen product to stabilize diluted bitumen streams.